Field of the Invention
The present invention relates to a dynamic microphone unit, and especially relates to a dynamic microphone unit which prevents that unevenness in frequency response is caused by resonance between an air chamber formed on a back surface of a diaphragm and an acoustic mass formed to a magnetic gap portion, and a dynamic microphone using the dynamic microphone unit.
Description of the Related Art
A non-directivity component of a dynamic microphone is resistance control. Therefore, an acoustic resistance is arranged on a back surface side of a diaphragm, and a back surface of the diaphragm is connected to a back side air chamber via the acoustic resistance. Accordingly, resistance control is realized by the acoustic resistance.
FIG. 6 is a sectional view illustrating an example of a conventional dynamic microphone unit. A reference sign 1 denotes a magnetic circuit. A disc-shaped magnet 2 is provided at a center of the magnetic circuit 1, and a disc-shaped pole piece 3 is arranged so as to come into contact with one side of magnetic poles of the magnet 2.
Further, a tail york 4 is provided so as to come into contact with another side of magnetic poles of the magnet 2. A peripheral edge of the tail york 4 is annularly erected, and a disc-shaped magnetic gap G is formed between an inner peripheral surface of the erected portion and a peripheral edge surface of the pole piece 3.
Through-holes 5 are concentrically formed so as to penetrate a center of the pole piece 3, the magnet 2, and the tail york 4.
The magnetic circuit 1 including the magnet 2, the pole piece 3, and the tail york 4 is attached to a unit case 6 supporting the tail york 4. A diaphragm 7 is attached to a front surface of an opening edge of the unit case 6.
The diaphragm 7 includes a center dome 7a and a sub dome 7b. A front surface of the center dome 7a is projected in a hemisphere shape. The sub dome 7b is formed annularly along a peripheral edge of the center dome 7a, and a front surface of the sub dome 7b is formed so as to project in an arc shape. A voice coil 8 is fixed to the diaphragm 7, for example, by using an adhesive at a boundary portion between the center dome 7a and the sub dome 7b on a back surface side of the diaphragm 7.
A peripheral edge of the sub dome 7b is attached to an opening edge of the unit case 6. The voice coil 8 is positioned in the magnetic gap G in such a state. When a sound wave is received in the configuration, the center dome 7a and the voice coil 8 can integrally vibrate in a front-back direction around an outer peripheral edge of the sub dome 7b by a sound pressure of the sound wave.
Accordingly, the voice coil 8 crosses a magnetic field generated in the magnetic gap G and outputs an audio signal based on vibration of the diaphragm 7.
Also, an equalizer 9 also functioning as a protective member for the diaphragm 7 is attached on an outer peripheral surface of a front edge of the unit case 6 so as to cover the unit case 6 and the diaphragm 7. A surface opposing to the center dome 7a at a center of the equalizer 9 is formed in a spherical shape which is recessed so that a fixed gap between the center dome 7a is kept.
Further, an opening 9a is formed at a center of the equalizer 9, and multiple openings 9b are formed along a periphery of the equalizer 9, to introduce a sound wave from the outside into the diaphragm 7.
A back surface side of the unit case 6 opens in a cylindrical shape, and a container-like lid 11 is attached by fitting to the cylindrical opening and closes a back surface of the unit case 6. In this manner, a back side air chamber 12 with a relatively large volume is formed in the container-like lid 11. The back side air chamber 12 is formed on a back side (side opposite to the diaphragm 7) of the magnetic circuit 1.
On the other hand, a volume reducing member 13 formed in a lens shape is arranged so as to oppose to a back surface of the center dome 7a. The volume reducing member 13 is attached, for example, by an adhesive to the pole piece 3 included in the magnetic circuit 1. A front surface of the volume reducing member 13 is projected in a spherical shape along a back surface of the center dome 7a. 
At a center of the volume reducing member 13, a through-hole (denoted by a reference sign 5 as with the through-holes of the magnetic circuit) is formed concentrically with the through-holes 5 formed to the magnet 2, the pole piece 3, and the tail york 4, which are included in the magnetic circuit 1.
Thus, a back surface of the diaphragm 7 is communicated with the back side air chamber 12 formed in the container-like lid 11 via the through-hole 5.
An acoustic resistance body 14 is attached to the through-hole 5 of the volume reducing member 13.
The acoustic resistance body 14 illustrated in the example is formed in a sheet-like shape. Therefore, a planer recessed portion 13a is formed at a center of the volume reducing member 13, and the sheet-like acoustic resistance body 14 is attached by an adhesive by using the recessed portion 13a. 
The volume reducing member 13 is used to prevent that an air chamber with a small volume is formed between a back surface of the diaphragm 7, especially a back surface of the center dome 7a, and the pole piece 3 of the magnetic circuit 1.
Specifically, in the case where an air chamber with a small volume is formed between a back surface of the diaphragm 7 and the pole piece 3 included in the magnetic circuit 1, the air chamber works as an acoustic volume (C component).
On the other hand, as described above, the voice coil 8 is arranged in the magnetic gap G, and therefore, each of an acoustic resistance (R component) and an acoustic mass (L component) is formed on an inner side and an outer side of the voice coil 8.
Therefore, resonance occurs between an acoustic volume (C component) of the air chamber and an acoustic mass (L component) formed to the magnetic gap G, and unevenness in frequency response of a microphone unit is caused.
A resonance frequency at this time is preferably equal to or greater than an upper limit of a main sound collective band of a microphone unit. Therefore, the lens-shaped volume reducing member 13 is arranged on a front surface of the pole piece 3 to reduce the acoustic volume (C component), and the resonance frequency is preferably set out of the sound collective band.
JP 2013-55396 A, JP 2013-55397 A, and JP 2013-141189 A disclose a dynamic microphone unit, in which the lens-shaped volume reducing member 13 is arranged on a front surface of the magnetic circuit 1, and a back surface of the diaphragm 7 is communicated with the back side air chamber 12 via the through-hole 5 formed at a center of the volume reducing member 13, as described above.
In the dynamic microphone unit illustrated in FIG. 6, the lens-shaped volume reducing member 13 is arranged on a front surface of the pole piece 3, and a recessed portion 13a for adhering an acoustic resistance body 14 is formed at a center of the volume reducing member 13 although the configuration, in which an acoustic volume (C component) in an air chamber formed on a back surface of the center dome 7a is reduced, is applied.
Therefore, the recessed portion 13a still acts as an acoustic volume, and this acoustic volume acts with an acoustic mass (L component) formed to the magnetic gap G. Accordingly, resonance in a sound collective band of a microphone unit is still caused.